Disturbances in Ca2+ homeostasis have long been linked to arrhythmias. Timothy Syndrome (TS) is the only cell surface Ca2+ channel defect linked to arrhythmia in humans. A single point mutation on the first S6 domain of Cav1.2 causes the syndrome. On a biophysical level, TS causes the disruption of the voltage dependent component of inactivation in Cav1.2. This slowing of voltage dependent inactivation has been hypothesized to increase net inward current during repolarization thereby prolonging the QT interval in patients. TS is an extremely rare disorder, with less than 30 cases reported. However, recent work has suggested that that a similar kinetic defect in Cav1.2 inactivation can be acquired through chemicals that cause aberrant phosphorylation of the Cav1.2 protein in the same physical region and is predicted to occur in transplant patients undergoing chronic treatment with cyclosporin. Thus, a study of TS may provide an understanding of the general role of Ca2+ channel inactivation in repolarization and promote the understanding of other conditions which alter inactivation of CaV1.2 in heart. Little is known about the actual changes initiated by the TS defect or about the pathophysiology of Timothy syndrome. To better understand this syndrome, we have created a transgenic mouse of TS. This mouse displays QT prolongation. The purpose of this R21 exploratory research proposal is to obtain a characterization of this mouse as a model of the human cardiac TS, particularly with respect to the electrophysiology of calcium channel inactivation, altered calcium handling, changes in other repolarizing currents and the role of Mosaicism/Chimerism. Specific Aim 1: Characterize Calcium Channel Expression and Current Properties This specific aim will characterize basic cardiac properties of the L-type calcium channel in the transgenic TS mouse model. These properties include changes in the magnitude of L-type calcium channel current expression, Cav1.2 activation, inactivation, and recovery as well as the physical distribution of the Cav1.2 isoform. We will also test the pharmacological sensitivity of the TS Cav1.2 mutation to dihydropyridines and beta adrenergic stimulation. Specific Aim 2: Characterize Repolarizing Currents, Calcium Handling and Related Changes. Altered repolarization characteristics in TS mice as measured by QT prolongation are much larger than anticipated on the basis of predicted changes in L-type calcium channel kinetics. This aim will examine whether a parallel reduction in repolarizing currents occurs and examine potential changes in calcium handling mechanisms. Timothy Syndrome is a rare disease resulting from a single well defined genetic defect. Patients with Timothy Syndrome die at an average age of 2.5 years. In order to better understand and treat this disorder we will study a mouse in which the same genetic defect has been artificially introduced.